A known electrophotographic image forming apparatus is designed to form an image on a recording sheet by transferring a developer image formed on a developing device, e.g. a photosensitive drum, onto the recording sheet and fixing the transferred image on the recording sheet by heat.
In the electrophotographic image forming apparatus, a great amount of heat is generated by an image forming device including the fixing device and the developing device. Thus, to prevent a temperature of the image forming device from excessively rising, the image forming apparatus includes an exhaust fan configured to discharge hot air from the apparatus.
The image forming apparatus also includes a permeable filter in an exhaust duct for discharging hot air. The permeable filter is configured to prevent dust suspended in air, e.g. developer particles and paper dust, from being discharged along with the hot air.
The permeable filter is configured to remove dust by filtering the air.
To sufficiently remove dust from the hot air, which is discharged from the apparatus, by using a permeable filter, the permeable filter needs to be made of a tight woven fiber such that minute air passages formed in the permeable filter can be decreased in size. However, if the capability of removing dust is improved in this manner, ventilation resistance at which the air passes through the permeable filter will increase.
When the ventilation resistance increases, the amount of air passing through the permeable filter may decrease, a sufficient amount of hot air may not be discharged, and the temperature in the image forming device may excessively rise.
Such problems may be solved with an exhaust fan having a high ventilation capability. However, to increase the ventilation capability, some measures may be taken, e.g., the exhaust fan may be increased in size or a rotational speed of the exhaust fan may be increased.
However, if a larger-sized exhaust fan is used, the image forming apparatus tends to become larger in size. If the rotational speed of the exhaust fan is increased, noise produced by the exhaust fan may become high.